The usage of area x-ray beams is common in the field of x-ray imaging and external radiation treatment to generate images of a target volume. For many imaging applications, a beam of x-rays or other radiation particles is directed from a radiation source and through a target volume or object. The traversing particles of the beam are collected in an imager or detector positioned on the far side of the target volume with respect to the source, and the data acquired by the acquisition (reception) of the particles in the imager may be subsequently used to generate an image of the target volume.
Certain characteristics of the particular beam used in acquiring the images are also critical to many applications, and may have a significant impact on the quality of the images and the treatment generated by the X-ray beams. For example, many digital X-ray imaging applications which include conventional scatter correction and dose verification techniques require knowledge of the spatial distribution of the flux intensity (also known as the beam profile) of the radiation beam used during the application. In addition, the beam profile data can also be used to facilitate calibration for X-ray detectors (e.g., gain calibration), and flat panel detectors in particular. Unfortunately, the generation of non-uniform beam fields is common in many radiology and medical imaging devices. This non-uniformity, when not properly calibrated for, can cause unintended artifacts in generated images which may significantly impact the quality of a generated image.
Non-uniformity of the beam field can be attributed to a variety of factors which include non-uniform directionality of Brehmsstrahlung radiation and the varying attenuation properties of the x-ray tube components (e.g. the Heel effect) and its associated housing. Ideally, in an ideal imager where the detector pixels each have a uniform response, the beam profile should approximate a flood field image (e.g., no object in the path of the beam). In practice however, practically all detectors also have gain variations caused by inherent imperfections in the sensors and the associated electronic circuitry. These gain variations must be calibrated out to create images that are sufficiently uniform and artifact-free. Unfortunately, conventional gain calibration techniques also remove flux intensity variations. Thus, after calibration, the flood field measurement will be a uniform signal, rendering any information regarding flux intensity variations lost.
One conventional approach of determining the beam profile involves performing a raster scan of the beam field with a small (e.g., single point) detector, determining the flux intensity of the beam at each designated point, and subsequently combining the results to form the beam profile. However, this approach suffers from several significant drawbacks: the scanning mechanism is cumbersome and the measurement time required is often lengthy since many beam pulses are required. Moreover, geometric pointing inaccuracies and global flux intensity variations from scan-to-scan can limit the accuracy of the final result. Another conventional method of determining the beam profile is by measuring the beam field using a large area of a detector. While more time-efficient than using a single point of a detector, the inherent non-uniformity among one or more pixels of the x-ray detector can detrimentally affect the accuracy of a beam profile thus measured.